Before I retired, I was ASE-certified in several categories, a member of SAE, and held technical and sales positions with Mobil and Dana. I also grew up in a large, full-service auto shop. I was taught that oil should be drained, rather than siphoned out of the dipstick tube, to enhance removal of sludge, metallic particles and other contaminants. If not, persistent removal of used oil without pulling the drain plug would endanger the life of the engine. A major retailer (Wal-Mart) is now installing siphon-type oil-changing equipment in its auto centers and will not remove the drain plug. This might solve a problem with untrained employees leaving the plug out, but it sure won't save much time. What are your opinions on this? -- Gary

RAY: Well, first of all, we don't know what Wal-Mart's plans are. So we'll limit our comments to the oil-change methods themselves.

TOM: We tend to agree with you, Gary. You do get more oil -- and, therefore, more crud and contaminants -- out of the engine when you remove the drain plug at the bottom of the engine and let gravity do its job.

RAY: The devices that pump or suck out the oil via the dipstick tube certainly are adequate, but not quite as good, in our humble opinion.

TOM: The places that use suction devices don't do it just to save time, they do it to decrease liability. When you -- as a mechanic -- remove someone's drain plug from the oil pan, there's always the possibility that you might (A) strip it, (B) cross-thread it, (C) undertighten it, (D) overtighten it, (E) lose the gasket, (F) damage the gasket or (G) drop the drain plug into your bowl of black-bean soup and never see it again. If any of those things happens, the consequences for the car can be catastrophic.

RAY: And if the consequences for the car are catastrophic, the consequences for the oil changer's bank account, or his insurance company's bank account, are going to be equally catastrophic. That's what the places that use the dipstick method want to avoid.

TOM: And you, as a car owner, want to avoid that, too. After all, even if you eventually got the place that forgot to tighten your oil plug to pay for rebuilding your engine, it would only be after an enormous hassle, and maybe not until after a small-claims-court appearance, and/or a threatening visit by a rented high-school football linebacker squad.

RAY: Now, there is another type of "oil suction" machine that we DO like. It's called an "engine flush," and it attaches where the oil filter is screwed in. It's an automated machine that uses high pressure to push out the old oil, clean the engine with a detergent and then refill the engine with new oil. That machine is very good, and it also limits liability. But it's designed for an occasional cleaning of engines that are thought to be sludged up. It's not for regular oil changes.

TOM: So, is using a regular suction device every bit as good as draining the oil via the drain plug? No. We don't think so. But if it's done right, done as thoroughly as possible and done while the engine oil is still warm, it probably does an adequate job.

RAY: Its biggest advantage is convenience. If it allows you to get your oil changed cheaply, while you're picking up 16 pallets of Chinese-made, recycled toilet paper, it might encourage people who otherwise wouldn't change their oil regularly to change it more often. That has to be factored into the equation, even if the oil-change method isn't quite as good.